Fizeau interferometers include a reference optic along the same optical pathway traveled by coherent light used for both illuminating and imaging a test object. A reference surface of the reference optic, generally referred to as a Fizeau surface, reflects a portion of the coherent light directed toward the test object as a reference beam and transmits another portion of the coherent light both to and from a test surface of the test object as an object beam.
The coherent light approaching the reference optic and test object is generally collimated, and in many Fizeau interferometers, the reference and test surfaces are oriented at least nominally normal to the approaching light for reflecting the light back towards its source. Unlike the reference surface, which is generally made both smooth and flat for retroreflecting light in the same collimated form at which the light approaches the reference surface, the test surface can be diffuse or include surface features as well as surface irregularities that are generally the subject of the measurement. Since the coherent light reflects from the test surface through a range of directions, an imaging system is used to collect the reflected light from both the reference and test surfaces. The imaging system images both surfaces onto a common imaging plane on which interference patterns form as a result of phase differences between the coherent light reflected from the reference and test surfaces.
These phase differences alone can be used to measure optical path length differences between the reference and test surfaces within an ambiguity interval of less than one wavelength of the coherent light. For measuring larger path length differences associated with test surfaces having larger features or irregularities, frequency shifting interferometers vary the frequency of the coherent light and monitor associated rates of change in phase to measure a much greater range of optical path differences between the reference and test surfaces. Since the test surfaces measurable by frequency scanning interferometers can include a much wider range of variation, greater demands are also placed on the imaging system to form aberration-free images of the test surfaces.
To accommodate the different depths at which the irregular features of test surfaces must be imaged without distortion, the imaging systems are generally arranged as telecentric imaging systems at least on the object side. Thus, the objective optics through which the reference and test surfaces are both illuminated and imaged must be larger than the field of view through which the measurements are taken. Conversely, the size of the objective optics determines the maximum size of the field of view subject to measurement. Considering that the field of view is generally set as large as practically possible to measure largest possible test objects or at least largest possible portions of even larger test objects, the objective optics themselves are often made as large as practically possible.
Most Fizeau interferometers include refractive objective optics located just in advance of the reference optic for both collimating the coherent light approaching the reference and test surfaces and for collecting and focusing the coherent light reflected from the reference and test surfaces through a telecentric stop. Given an ongoing demand for limiting aberrations, large objective optics tend to be difficult and expensive to manufacture and, as a matter of practicality, limited in size and weight.
The objective optics are also known to produce so-called “hot spots” near the optical axis where the objective optics themselves tend to retroreflect portions of the coherent light along the same path as the returning reference and object beams. The retroreflected light from the objective optics reduces phase contrast near the center of the image plane.